WARNING: This article contains major spoilers for "Deus Lex Machina," the latest episode of Supergirl.
Supergirl's adoptive father Jeremiah Danvers didn't die of a heart attack. He was murdered.
In "Deus Lex Machina," the latest episode of Supergirl, Lex Luthor revealed his master plan. Part of that involved Eve Teschmacher, who he "rescued" from Leviathan. In order to get leverage over her, he manipulated her into killing Jeremiah under the pretense that Jeremiah was the Leviathan operative who killed her father.
After he pulled off the last move in his plan, which involved pitting Supergirl and Leviathan against one another, Lex returned home to find Eve waiting with dinner. She was overjoyed his plan had worked and admitted she was excited to spend time as a couple, but Lex pulled her up short. He revealed she had distracted him from his plan in a previous timeline and, because of that, he couldn't love her.
"I saved you because you were useful," Lex explained. "I needed a spy inside Leviathan. As for your mother? She's still in danger, but from me. Those cars I showed you? The men in them are just waiting to pull the trigger if you ever betray me, but I'd think twice about that."
"That man you killed in the Andes didn't murder your father," he continued with a devious smile. "I never bothered to find out who knocked off your old man! No, the man you murdered is Supergirl's father. I have video of you killing him, by the way. Defy me and I'll pass it on. Believe me, you do not want to be on the wrong side of those laser eyes."
As such, Eve is stuck between a rock and a hard place. She can't turn Lex into the authorities, on account of all the crimes she committed under his purview. She certainty won't want to ask Supergirl for help, since she poisoned her father to death, albeit unknowingly. This could, however, leader her to Lena Luthor, who has an uneasy alliance with her brother at best.
In the previous episode, Jeremiah's daughter Alex and Kara were told he suffered cardiac arrest and died. After escaping Cadmus Labs, he hid out in the Peruvian Andes, where he did refugee work with displaced aliens. His death sent Alex into a tailspin, which led her to use Leviathan's Obsidian lenses to escape her pain. This caused her to become stuck in the Obsidian VR landscape, which -- in turn -- made Team Supergirl shift their focus to Leviathan instead of Lex.
As played by Lois & Clark star Dean Cain, Jeremiah was introduced as the Danvers family patriarch in the Supergirl pilot. His work with the DEO led him to meet Superman, who eventually trusted his family to raise Kara. In pre-"Crisis on Infinite Earths" continuity, he participated in a mission that led him to meet J'onn J'onzz. He realized the Martian Manhunter was a friend and not a foe, but could not convince his superior Hank Henshaw of that. He fought Henshaw to protect J'onn, but was mortally wounded in the process. He begged J'onn to watch over his daughters and died.
However, that was not the last of Jeremiah. As it turns out, Project Cadmus recovered his body and experimented on him. The organization then blackmailed him into working with them by threatening to hurt Kara and Alex. As a result, he was blacklisted by the DEO and labeled a threat. When push came to shove, though, he betrayed Cadmus to help Alex. He was kidnapped by them after the resulting conflict and never seen again. He last appeared in the Season 2 episode "Exodus."
Airing Sundays at 9 p.m. ET/PT on The CW, Supergirl stars Melissa Benoist as Kara Danvers, David Harewood as Martian Manhunter and Chyler Leigh as Alex Danvers.